When a measurement over a predetermined range of mass-to-charge ratios (m/z) is performed in a mass spectrometer, a profile spectrum is normally obtained as a measured waveform which shows the change in the ion intensity over that mass-to-charge-ratio range. Typically, in this profile spectrum, the intensity signal which corresponds to an ion originating from one substance emerges in the form of a peak having a certain mass-to-charge-ratio width. Accordingly, centroid processing for calculating the centroid of each peak on the profile spectrum is generally performed to determine the mass-to-charge-ratio value corresponding to the ion and create a mass spectrum in which a linear peak is drawn at the position of the determined mass-to-charge-ratio value (see Patent Literature 1 or Non Patent Literature 1).
A technique which employs trapezoidal approximation is frequently used as an algorithm for calculating the centroid position of the peak (i.e. m/z value) and the peak intensity value in the centroid processing. FIG. 7 is a schematic diagram illustrating this algorithm.
In FIG. 7, the points Pr indicated by the black dots (•) are data actually obtained by a measurement. Actual measurement data Pr which are adjacent to each other on the mass-to-charge-ratio axis are connected by line segments, and each intersection point Pt is determined between the line segments and the threshold level which is determined based on the peak-top value, i.e. the largest value among the actual measurement data values forming one peak. As shown in FIG. 7, the intersection point Pt is located on each of the rising and falling sides of the peak. A perpendicular line is dropped from each of the two intersection points Pt on the threshold line as well as each of the actual measurement data Pr equal to or higher than the threshold, onto the baseline, to determine the point of intersection of each perpendicular line and the baseline. A plurality of trapezoids are thereby formed, with their parallel sides formed by the perpendicular lines and their non-parallel sides formed by the baseline and the line segments connecting the actual measurement data Pr. A total of five trapezoids are obtained in the example of FIG. 7. The centroid of the polygon obtained by combining those trapezoids is calculated, and the value of the mass-to-charge ratio at which the centroid of the polygon is located is chosen as the centroid mass-to-charge-ratio value. Additionally, the centroid intensity value is calculated by totaling the areas of the trapezoids.
For example, in a liquid chromatograph ion trap time-of-flight mass spectrometer (LC-IT-TOFMS) in which a liquid chromatograph (LC) or other types of chromatograph is combined with an ion trap time-of-flight mass spectrometer, if all compounds separated by the LC and successively introduced into the IT-TOFMS must be completely detected, a measurement covering a predetermined time-of-flight range (i.e. a predetermined mass-to-charge-ratio range) must be repeated at short intervals of time in the IT-TOFMS. In that case, the duration of one peak on the time-of-flight spectrum obtained by a single measurement in the IT-TOFMS becomes short, and accordingly, the number of points of the actual measurement data forming one peak becomes small.